Wear resistance of piston ring of a car engine is examined in a test drive by adding a small amount of radioactive iron isotope ⁵⁹Fe with half-life of 45 days. In the beginning of the test drive piston ring mass was 100.0 g and its activity was 9.4Ci (1 Ci = 3.70 x 10¹⁰ Bq). Engine is tested for 1000.0 hours after which motor oil is poured off and measure for activity. Oil activity was measured to be 84 decays per second.

Calculate how much the piston ring's mass was reduced during the test drive.